The switching-mode power conversion technology has over the years radically changed the appearance of commercial products, making them far smaller than a few decades ago and leaving the designers with possibilities to experiment with their look and feel without being limited by technology barriers. These benefits are a direct result of the improved efficiency of the power supplies and power amplifiers that use a switching approach instead of a linear one. As a side effect, the amount of heat-sinking material needed is reduced by at least an order of magnitude, which improves the level of integration between the various components, so that the overall board space, weight and volume are significantly reduced and power density is improved. The aforementioned advantages are probably most clearly seen in the switching-mode Class D audio power amplifiers, where the new efficient power conversion principle has opened the doors to some new and challenging application areas, from the smallest low-end portable devices with extended battery life to the large high-end audio installations for stage performances with tremendously reduced dimensions.
The achievements in the field of switching-mode audio power amplification in the last few years, described in terms of even higher output power levels and improved audio performance, are drawing this approach on the technology map as one of the most significant breakthroughs that is eventually going to replace linear electronic in most of the power processing applications. However, this does not mean that the present Class D audio power amplifiers are the only possible solution that fits all application, and much research is done to take the most advantage from the very high conversion efficiency of the switching-mode approach while still keeping the complexity and component count to a minimum.
These unique challenges, posed predominantly by the audio and video product manufacturers wanting to penetrate the low-end market by cutting production costs and introducing cheap products of satisfactory quality, can be answered by further and closer integration of the constitutive parts of the audio power amplifier, i.e. the switching-mode power supply and the Class D amplifier, which until now usually have been viewed as separate parts without many touching points.
Several initiatives to improve power conversion system using a control approach that utilises the self-oscillating principle have been proposed.
DE 198 38 765 discloses a power amplifier employing a hysteresis control for generating pulse width-demodulated voltages. The difference between the input voltage and the output voltage is integrated in this power amplifier, said difference being stepped down by a factor corresponding to the ratio of the maximum level of the input voltage to the maximum level of the output voltage. The difference between the scaled output signal and the input signal corresponds to the instantaneous amplitude error of the output signal with the result that the integration corresponds to the accumulated error on the output. The output signal of the integrator is triangular, and when the power amplifier is idle running, the slope of said triangle is of the same value for both the positive and the negative flanks. When the power amplifier is to be set, i.e. loaded, these flanks change in such a manner that the positive flank discloses a slope differing from the slope of the negative flank. However, the curve shape remains triangular with straight flanks. As the power amplifier is increasingly loaded, the switch-frequency decreases as well. As a result, for instance the input signal to the power amplifier is sinusoidal, and then the switch-frequency is at maximum at the zero-pass for the sine curve and significantly lower at the maximum and the minimum value, respectively, of said sine curve. When the power amplifier is loaded to its maximum, i.e. when the maximum value of the output voltage is almost identical with the internal DC-voltage of the power amplifier, then the switch-frequency becomes very low, almost zero. The triangular signal from the integrator is transferred to a comparator, typically a Schmitt-trigger, which converts the triangular signal into square pulses of a varying pulse width. These square pulses are the switching on signals and the switching off signals, respectively, for the transistors in the power amplifier. These switching on pulses are transferred to the output stage of the power amplifier, viz. to the transistors in the output, and therefore these pulses are upscaled by the relation between said pulse voltages and the internal DC-voltage of the power amplifier. The resulting voltage includes square pulses and is typically of a higher amplitude than the signal voltage. The square voltage is then transmitted to the output filter of the power amplifier, said output filter typically being a second order filter which is often referred to as a reconstruction filter. The voltage applying after the filter is the output voltage of the power amplifier. The voltage returned to the integrator is the voltage applying before the output filter. A modulator of this type is often referred to as an Astable Integrating Modulator or an AIM. Such a modulator is encumbered with the problem that the distortions of the output filter have not been taken into account. In addition, the operational amplifier used to construct the integrator has to be of high quality.
WO 02/25357 discloses a controlled oscillation modulator, also called a COM. The COM ensures that the open-loop-phase characteristics involve a phase shift of exactly 180° at the frequency where the open-loop-amplification is 0 dB. The latter is rendered possible by the feedback voltage from the output stage of the power amplifier being forwarded through function blocks causing a phase shift of 180° and/or through function blocks with time delays. The desired phase shift of 180° is obtained by including said phase shift in the function blocks, such as in form of a cascade coupling of poles, and/or by choosing a suitable time delay. When the feedback loop is subsequently closed, the modulator oscillates at the frequency, where the amplification is 0 dB. When the input signal to the power amplifier is 0, the resulting signal is a substantially pure sine. When the input signal differs from 0, the oscillation is superimposed by the input signal. A comparator is subsequently used for generating the switching pulses of the output stage. An increasing loading of the amplifier has the effect that the pure sine resulting from the phase shift of 180° is altered into being something between the pure sine and the triangular voltage known from AIM. The linearity of a modulator depends on variations in the inclination of this signal. As this signal is not a pure triangular curve unlike AIM, but instead something between a sine curve and a triangular curve, the modulator according to the COM principle is nonlinear, and the modulation per se distorts the output signal.
WO 98/19391 describes a way of improving a class D power amplifier. The amplifier includes an internal modulator generating the well-known pulse-width-demodulated output signal. This signal is transmitted to an output filter, and the resulting filtered signal is the output voltage of the power amplifier. In order to compensate for the distortions of the filter, additional feedback loops have been included, and the characteristics of these feedback loops can compensate for the distortions of said output filter. The described system includes several cascade-coupled feedback loops for compensating the distortions. The system shows an improved procedure structure with respect to power amplifiers without such feedbacks, but the system is per se very complex and requires much design work in order to achieve the desired effect. A system of this type is often referred to as being Multivariable Enhanced Cascade Controlled or MECC.
WO 04/100356 discloses a switch-mode modulator operating at a two-level voltage and including an alternating output stage, an optional output filter and a feedback including a function block with a transfer function. The modulator furthermore includes a forward block provided with means for calculating the difference between the signal originating from the function block and a reference signal as well as with a transfer function. The output of the forward block is the input of a Schmitt-trigger, which generates switch on signals for changing the output stage. The output voltage of the modulator applying either after the optional output filter or the output stage is fed back through the function block so as to generate the signal fed back. The transfer function of the function block and of the forward block is chosen both in response to the transfer function of the output filter and in response to the desired total open-loop-transfer function of the modulator.
U.S. Pat. No. 4,992,751 discloses an audio amplifier with phase modulated pulse width modulation, wherein clamping means are used for removing overshoot spikes in the pulse width modulated output signal.
U.S. Pat. No. 6,552,606 B1 discloses a modulator in which the feedback is the current measured through the capacitor of the output filter. The power amplifier is thus a voltage controlled current generator.
All the above mentioned modulators are supplied by a DC-link converter.
HF-link converters represent an interesting alternative to the conventional isolated power converters with DC-link as found in a wide range of products from audio power amplifiers, converters for renewable energy sources, uninterruptible power supplies (UPSs) to general isolated power converters. They feature compact design achieved through the removal of the DC-link and the bulky filters associated with it, leading to a high level of integration, higher efficiency, less volume and board space, low component count and subsequently low cost.
However, the continuously changing polarity of the HF-link voltage causes the prior art self-oscillating modulators for DC-link converters to become unstable and therefore unusable in HF-link converters.
The object of the invention is to provide a new and improved self-oscillating modulator, which is especially applicable for HF-link converters.